Answer 1

Hi,
This is a very generic question and difficult to answer without more details of your project.
An expert system can mean many things - in essence it would give the user some recommendation based on certain input parameters.
Based on your profile you are already familiar with html and php and SQL so that could be a good start. Implementing a simple decision tree is not difficult in JavaScript, and tapping into a database can be done with a bit of php.

If you are looking for more advanced algorythms, like machine learning and predictive modeling, then I would recommend to get familiar with python, but be prepared for a longer learning curve and advanced mathematical and statistical knowledge is also very useful for this direction.